1. Field of the Invention
This invention relates generally to integrated circuits which produce clock frequencies. Specifically, the present invention provides for inter-related calibration techniques for a precision relaxation oscillator that produces a stable clock frequency over wide variations of ambient temperature, fabrication process and voltage. The invention is implemented on a single, monolithic integrated circuit.
2. Description of the Prior Art
The current state of the art describes RC relaxation oscillators which primarily depend on one of two schemes. In the first example as found in FIG. 1, a single comparator is coupled to a pulse generator to alternately charge and discharge a capacitor to produce a clock for a xe2x80x9cD typexe2x80x9d flip-flop. Several error sources are present in this design. The resistor and capacitor typically have unpredictable voltage and temperature coefficients. The charging current and comparator input slew are a function of the supply voltage which is also subject to drift. Also, the pulse generator output may vary with temperature and supply voltage. These factors lead to a clock frequency that varies over temperature.
In a second example as illustrated in FIG. 2, an RC circuit provides a common input to each of two comparators. Independent reference voltages are coupled to each of the remaining inputs of the comparators. The outputs of each of the two comparators are coupled to the inputs of a xe2x80x9cSet-Reset typexe2x80x9d flip-flop. The output of the flip-flop serves to alternately charge and discharge the capacitor. Although this circuit eliminates the inaccuracies of the pulse generator as discussed above in FIG. 1, other problems manifest themselves. A duty cycle error may occur since it is unlikely that the capacitor will charge and discharge at the same rate, especially over temperature variations. Also, error is induced by the difficulty of providing two reference voltages which track each other coincidently over temperature.
Therefore, a need existed to provide a relaxation oscillator which is capable of maintaining a stable clock frequency independent of temperature. Furthermore, a need existed to provide a calibration technique for trimming the relaxation oscillator for a target clock frequency to compensate for process variations.
It is the objective of the present invention to provide several calibration techniques which all have the common goal of providing a capacitor charging current for achieving a target clock frequency and for optimizing a temperature coefficient of the relaxation oscillator circuit.
It is an object of the present invention to provide an independent current calibration technique which independently sets CTAT current to a pre-determined value and sets PTAT current to zero or approximately zero. In at least one embodiment, the PTAT current generator may be omitted from the circuit altogether. In a variation of this calibration technique, it is the object of the invention to independently set PTAT current to a pre-determined value and set CTAT current to zero or approximately zero. In this variation, the CTAT current generator may be omitted. It is further the object of the independent current calibration technique that the CTAT (or PTAT) calibration values be stored in nonvolatile memory.
It is another object of the present invention to provide a fixed compensation calibration technique which computes a mean PTAT current for a population of devices from a random sample of the population, computes the PTAT calibration values for determining the mean PTAT current in a device exhibiting mean performance and applying those PTAT calibration values to the entire population of devices. It is further the object of the fixed compensation calibration technique that the CTAT calibration values be stored in non-volatile memory and the PTAT calibration values be hardwired to appropriate logic levels.
It is another object of the present invention to provide a variable compensation calibration technique for a precision oscillator with temperature compensation which computes a mean PTAT current for a population of devices from a random sample of the population, computes the PTAT calibration values for determining the mean PTAT current in each device within the population and applies the individually computed PTAT calibration values to each device in the population. It is further the object of the variable compensation calibration technique that the CTAT and PTAT calibration values be stored in non-volatile memory.
It is another object of the present invention to provide a fixed ratio calibration technique for a precision oscillator with temperature compensation which computes a mean PTAT:CTAT current ratio for a population of devices from a random sample of the population, computes the PTAT and CTAT calibration values for determining the mean PTAT current in a device exhibiting mean performance and applying those PTAT and CTAT calibration values to the entire population of devices. It is further the object of the fixed ratio calibration technique that the CTAT calibration values be stored in non-volatile memory and that the PTAT calibration values be decoded from the CTAT calibration values.
It is another object of the present invention to provide a variable ratio calibration technique for a precision oscillator with temperature compensation which computes the temperature coefficients for the PTAT current generator and the CTAT current generator by taking current measurements at two distinct temperatures for each device in the population, deriving the PTAT:CTAT current ratio from the ratio of the temperature coefficients, computing the PTAT and CTAT calibration values for determining the PTAT and CTAT currents in each device and applying the independently calculated PTAT and CTAT calibration values to each device in the population. It is further the object of the variable ratio calibration technique that the CTAT and PTAT calibration values be stored in non-volatile memory.
In accordance with one embodiment of the present invention, a precision relaxation oscillator that produces a stable clock frequency over wide variations of ambient temperature is disclosed. The precision relaxation oscillator is comprised of an oscillation generator, a first current generator for producing a first output current and a second current generator for producing a second output current. The invention is implemented on a single, monolithic integrated circuit.
In accordance with the independent current calibration method of the present invention, the CTAT current is set to a predetermined value at a nominal temperature and PTAT current is set to zero or approximately zero. Alternatively, the PTAT current is set to a predetermined value at a nominal temperature and CTAT current is set to zero or approximately zero.
In accordance with the fixed compensation method of the present invention, the mean PTAT current is computed for a random sample of a population of devices at a nominal temperature. The calibration values for setting the PTAT calibration select switches are computed for setting mean PTAT current in those devices exhibiting mean performance. The computed calibration values are stored in each device of the population. The CTAT calibration values are computed on a device by device basis for achieving a CTAT current such that the sum of the PTAT current and the CTAT current corresponds to a capacitor charging current for a target clock frequency.
In accordance with variable compensation method of the present invention, the mean PTAT current is computed for a random sample of a population of devices at a nominal temperature. The calibration values for setting the PTAT calibration select switches are individually computed for setting mean PTAT current in each device of the population. The CTAT calibration values are computed on a device by device basis for achieving a CTAT current such that the sum of the PTAT current and the CTAT current corresponds to a capacitor charging current for a target clock frequency.
In accordance with fixed ratio method of the present invention, the mean ratio of PTAT:CTAT current is computed for a random sample of a population of devices at a nominal temperature. The calibration values for setting the PTAT and CTAT calibration select switches are individually computed for setting the mean ratio of PTAT:CTAT current in each device of the population.
In accordance with variable ratio method of the present invention, the temperature coefficients for the PTAT current generator and the CTAT current generator are computed by taking current measurements at two distinct temperatures for each device in the population. The PTAT:CTAT current ratio for each device is derived from the ratio of the temperature coefficients. The PTAT and CTAT calibration values for determining the PTAT and CTAT currents in each device are computed and applied to each device in the population.
The foregoing and other objects, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.